The present invention is principally concerned with the machining of metals using single point cutting tools. With such tools, as with most metal cutting in general, a coolant is applied to the vicinity where the cutting takes place in order to improve tool life and enhance the quality of the surface finish. Generally, the coolant also has lubricating properties; a combination of water soluble oil and water is often favored because of cost and good heat transfer characteristics. In certain instances mineral oils and other coolants are utilized.
The dynamics of what occurs at the interface between the edge of the cutting tool and the workpiece have been extensively investigated, according to the literature. This is attributable to the significant economic impact which can result from improvements in cutting tools and cutting methods. In recent years, a major development has been the prolongation of tool life by the use of sintered metal carbides instead of hardened alloy steels. But carbide tools still degrade with time and it is of great interest that this be lessened. Benefits which accrue from extending tool life are much greater than the replacement cost of the tool. They include the avoidance of down time, cost of recalibrating the tool location, and avoidance of possible damage to the workpiece which can occur when a tool breaks unexpectedly.
In machining generally, the chip which is cut from the workpiece strikes the rake surface of the cutting tool and thereby both causes mechanical wear and transfers heat to the cutting tool. Damage to this surface and the cutting edge itself will ultimately cause the tool to break away, unless the tool is preventively replaced. Thus, it has been logically sensed that the tool cutting edge and the top surface of the tool where the chip strikes are the regions most in need of coolant. Therefore, earlier inventors took steps to specifically direct the coolant to such a location, as is shown for instance in U.S. Pat. No. 2,744,451 to Lee.
Generally, such "flood coolant" systems are most often used even today. Coolant is circulated by low pressure pumps, with outputs of the order of 70-105 kPa. In flood cooling it is only required to draw the coolant from the machine sump, lift it to a certain height above the tool and to then generally discharge it in the vicinity of the workpiece and cutter. Of course, the coolant which lands on the workpiece or cutter, whichever may be moving, is thrown about. Shields are used to contain the coolant which flies from the vicinity of the cut. Generally, it has been undesired to add to this spray of coolant by having excess pressure at the point of discharge of the coolant. Thus, the circulating pumps used in applying coolant have been low pressure pumps and high coolant discharge nozzle pressures have been avoided. In fact, "mist cooling", comprising the use of a stream of air and water droplets, is commonly used in less severe situations.
Even as long as 90 years ago it was recognized that there could be certain improvements in the manner in which the coolant is delivered to the vicinity of the tool cutting edge. For example, Chouteau in U.S. Pat. No. 522,588 shows coolant directed along channels in the rake surface of a tool bit. More recently, Pigott in U.S. Pat. No. 2,683,303 shows how coolant is directed from a manifold directly transverse to the cutting face of a milling cutter. Onsrud in U.S. Pat. No. 2,524,232 shows coolant delivered to the cutting face of a cutter by means of channels drilled in the body of the cutter, which channels discharge fluid immediately ahead of the cutting face. Jennings in U.S. Pat. No. 3,176,330 shows a carbide insert held in place by a chip breaker which has channels in it. The coolant is supplied to the chip breaker and discharges through the channels toward the cutting face.
While the foregoing art is representative of certain efforts which have been made, the aforementioned inventions have not been widely utilized. Most probably there are several reasons for this, including that insufficient improvement has been realized from them, that it is undesirable to cut channels in the surface of a cutter and that it is inconvenient to deliver coolant to nozzles that have to be mechanically integrated with the cutting tool. It has been simply both easier and sufficient to use a flood coolant procedure.
In the making of the present invention, low pressure coolant was initially directed specially at the cutting face of the tool, more or less along the lines taught by the Jennings patent. However, using this procedure only gave a certain inadequate tool life in the machining grooves on the outside diameter of titanium and nickel alloy cylinders. Both significant wear and premature breakage were encountered. To increase tool life further research was undertaken, and as a result the invention was made.